Stability and transport characteristics of Poiseuille-Rayleigh-Bénard double diffusive convection of binary fluids

To investigate the stability, heat and mass transfer characteristics of Poiseuille-Rayleigh-Bénard (P-R-B) double diffusive convection of binary fluids in horizontal channels, we conducted three-dimensional numerical simulations using the finite volume method. The ranges of spanwise aspect ratio (B) of the channel, Reynolds (Re) number, buoyancy ratio (N) and Rayleigh (Ra) number are 1 ≤ B ≤ 10, 3 ≤ Re ≤ 25, −0.3 ≤ N ≤ 0.3, and 1.1 × 10³≤Ra≤1.2 × 10⁵, respectively. Results reveal that when secondary flow forms, the fluid's temperature and concentration exhibit sinusoidal variations in the spanwise direction. The secondary flow is characterized by the longitudinal rolls (LRs). As B increases or Re decreases, the onset of LRs and instability occurs at a lower critical Ra. As N shifts from negative to positive, the critical values for the onset of LRs and instability decrease. The variations of both Nusselt (Nu) and Sherwood (Sh) numbers along the streamwise direction can be divided into three regions. Despite the influence of solute buoyancy, the behavior of Nu variation in binary fluids is similar to that in pure working fluids. Meanwhile, Nu and Sh consistently follow the same trend, regardless of change in the direction of the solute buoyancy force. The increases in Ra and N enhance heat and mass transfer. The influence of B on heat and mass transfer abilities depends on the interplay between the number of LRs and the non-uniformities of the temperature and concentration distributions. Lastly, correlations for heat and mass transfer in fully developed regions of P-R-B double diffusive convection of binary fluids have been derived.